In the practice of the well-known Bayer process, bauxite is digested in a sodium hydroxide-sodium aluminate liquor, unsaturated for sodium aluminate, at temperatures between 100.degree. C. and 300.degree. C., depending on its mineralogical composition, and the soluble aluminum minerals are transformed to sodium aluminate. After digestion, the slurry is cooled to the atmospheric boiling point. After dilution and removal of digestion residue (red mud), aluminum hydroxide (alumina hydrate) is precipitated out of a solution supersaturated for dissolved alumina by further cooling followed by separating the alumina hydrate precipitate. The mother liquor--possibly after concentrating a part of its by evaporation--is recycled to the digestion. After washing, the alumina hydrate is calcined to alumina, i.e. technically pure Al.sub.2 O.sub.3, at a temperature exceeding 1000.degree. C.
In addition to the digestible aluminum minerals (gibbsite, boehmite, diaspore) bauxites always contain some amounts of silicon-containing components. The SiO.sub.2 content occurs mainly as kaolinite (Al.sub.2 O.sub.3.2SiO.sub.2.2H.sub.2 O) but also as Quartz (SiO.sub.2), chamosite, illite, halloysite in the bauxites. Moreover Fe.sub.2 O.sub.3 - and TiO.sub.2 -containing components can also be found in bauxites. Bauxites containing at least 90% of the digestible aluminum minerals in the form of gibbsite are considered to be of the gibbsitic type.
It is well known from the technical literature that the silica-containing minerals found in the bauxites transform to sodium aluminum hydrosilicates during the Bayer process.
Various authors give values between 0.62 and 0.79 for the Na.sub.2 O to SiO.sub.2 mass ratio of the sodium aluminum hydrosilicates formed from reactive silica in solution of 80 to 200 g/l caustic Na.sub.2 O concentrations (135 to 350 g/l as Na.sub.2 CO.sub.3) and 2.5 to 3.5 caustic molar ratios (0.27 to 0.38 A/C ratios) as 140.degree. to 150.degree. C. temperatures. (Adamson, A.N.: Alumina Production: Principles and Practice. The Chemical Engineer, June 1970, pp. 156-171; Sartowski Z., Vargane Kiss Zs., Bulkai D.: Elokovasavtalanitas Bayer-eljaras ban. Banyaszati es Kohaszati Lapok, Kohaszat, 114, (1981), 2, pp. 79-85; Yamada, K. Et al.: Process for Extracting Alumina From Aluminous Ores. U.S. Pat. No. 4,426,363). This ratio makes it possible to calculate the chemical caustic soda loss, which is a significant cost component of the alumina manufacturing by Bayer process.
Caustic Na.sub.2 O is defined as the amount of Na.sup.+ ions equivalent to the sum of the free OH.sup.- ions and to the OH.sup.- ions present as complex aluminate anions e.g. [Al(OH).sub.4 ].sup.- in the liquid phase, expressed as Na.sub.2 O. The caustic molar ratio is the ratio of the caustic Na.sub.2 O and the Al.sub.2 O.sub.3 mols present in the solution. Any reference herein to "molar ratio", or "c. molar ratio" is meant to refer to the caustic molar ratio defined hereinabove. The use of this expression is most suitable in connection with the present invention, because it permits easy expression of the small aluminate concentration that is a unique characteristic of the present invention, without involving the actual caustic soda concentration that is employed.
In the American terminology, the caustic concentration--understood similarly--is expressed as Na.sub.2 CO.sub.3. Likewise, the ratio of the concentration of dissolved Al.sub.2 O.sub.3 to the caustic concentration expressed as Na.sub.2 CO.sub.3 is also widely used in American terminology, and it is called in the specification and the claim "A/C ratio".
Gibbsitic bauxites are usually digested at 140.degree. to 150.degree. C. (in some exceptional cases at about 105.degree. C.) by the so-called low-temperature digestion process. According to the typical practice, the bauxite is slurried in about 10% to 20% of the liquor recycled to the digestion, which is relatively poor in dissolved Al.sub.2 O.sub.3 (so-called spent liquor). The slurry--having a temperature of about 80.degree. C--is mixed, after a holding time usually not exceeding 1 to 2 hours and usually without further preheating, with the main flow of the digestion liquor preheated in heat exchangers to about 160.degree. C. By introducing live steam into the mixed slurry the prescribed digestion temperature of 140.degree. to 150.degree. C. is maintained. This briefly described process is called two-stream digestion.
Mother liquors with typical caustic Na.sub.2 O concentrations of 90 to 120 g/l (150 to 205 caustic as Na.sub.2 CO.sub.3) and typical molar ratios of 2.6 to 2.8 (0.34 to 0.37 A/C ratios), obtained after hydrate precipitation (called spent liquors in the technical literature dealing with alumina manufacturing), are used in the digestion of gibbsitic bauxites. The bauxite dosage into the digestion liquor (the bauxite to digestion liquor ratio) is controlled so that the caustic molar ratio of the liquor phase of the slurry leaving the digestion reactor should be 1.35 to 1.45 (its A/C ratio should be 0.66 to 0.71). A retention time of about 40 to 100 minutes is usually utilized in the low temperature digestion process. This is carried out in 3 to 5 series-connected digester vessels (autoclaves). Gibbsite is digested during the first 10 to 20 minutes of the digestion process.
Parallel to the reaction of the gibbsite, kaolinite also is dissolved into the liquid phase. The dissolved silica, probably present in the form of some complex ion, transforms into a solid-phase sodium aluminum hydrosilicate in a reaction leading to an equilibrium. A retention time longer than that required for the dissolving of gibbsite is necessary for the formation of nuclei and growth of the crystals. Therefore, retention times of 40 to 100 minutes are provided in the reactor (Carlos, S.: Interalumina Bauxite Grinding and Digestion. Engineering and Mining Journal, May, 1983, pp. 29-94; Kotte, J.J.: Bayer Digestion and Predigestion Desilication Reactor System. Light Metals. Proc. of AIME Annual Conference, 1981, pp. 45-79).
The liquid phase of the slurry, leaving the reactor at a temperature of 140.degree. to 150.degree. C. and with a molar ratio of 1.35 to 1.45 (0.66 to 0.71 A/C ratio), is supersaturated for boehmite. The ratio of the bauxite and the digestion liquor is controlled so that the difference between the actual Al.sub.2 O.sub.3 concentration of the liquor and the equilibrium value be not less than 18 to 20 g/l. Should this difference be less, the secondary boehmite formation amounting to 0.5% to 2% Al.sub.2 O.sub.3 during the usual retention time would increase and attain an extent unbearable from the economic point of view because of the increase of the supersaturation of the liquor for boehmite.
The purpose of the red mud washing is to minimize the caustic Na.sub.2 O and Al.sub.2 O.sub.3 concentrations of the liquor accompanying the red mud. In the settler the liquid phase is supersaturated for Al.sub.2 O.sub.3, so the precipitation of the equilibrium phase i.e. gibbsite begins. Due to the lower caustic Na.sub.2 O concentration and temperature in the first washing stage the supersaturation for Al.sub.2 O.sub.3 is higher than in the settler, so the secondary gibbsite formation is more intensive. Due to the secondary gibbsite formation, a loss amounting to 2% to 3% of the Al.sub.2 O.sub.3 content of the processed bauxite can be observed during the settling and washing of the red mud.
European Patent Application No. 203,873 deals with the processing of low-SiO.sub.2 gibbsitic bauxites. According to the process of Lepetit and Mordini, the bauxite is ground in a liquor of 50 to 120 g/l caustic Na.sub.2 O concentration (90 to 205 g/l as Na.sub.2 CO.sub.3) originating at least partially from the washing of red mud or alumina hydrate. The resulting slurry is treated at a temperature of 80.degree. to 100.degree. C. for such a time that at least 85% of the kaolinite is transformed into sodium aluminum hydrosilicate. The disadvantage of this process is that, as a result of the grinding in a wash water, the digestion is burdened with the feeding of excess water, the evaporation of which will increase the energy consumption of the processing.
A procedure for the processing of high-kaolinite-containing gibbsitic bauxites is proposed by Grubbs (U.S. Pat. No. 4,614,641). According to this invention the bauxite is crushed and subsequently classified at 105 .mu.m (150 mesh). The fine fraction, richer is kaolinite, is digested in a sodium aluminate liquor, having a caustic Na.sub.2 O concentration higher than 140 g/l (240 g/l as Na.sub.2 CO.sub.3), at a temperature of 80.degree. to 130.degree. C. The solid phase is removed by settling and the liquid phase, rich in aluminum and silica, is mixed with the coarse fraction digested in a separate stream at 140.degree. to 150.degree. C.
A procedure has been proposed by Grubbs, in U.S. Pat. No. 4,661,328 for the processing of gibbsitic bauxites having a high kaolinite content. According to Grubbs the dissolution of kaolinite is inversely related to the caustic concentration and silica concentration of the digestion liquor. The digestion liquor originally having a low silica concentration, is fed to a back mixed reactor in which the silica and alumina concentrations of the digestion liquor are increased to slightly below saturation levels. The bauxite has a reactive silica content of over 5% by weight, and the digestion liquor typically contains 1.8-2.5 g/l silica, 150-170 g/l alumina, and 240-300 g/l caustic soda expressed as calcium carbonate (140-175 g/l caustic Na.sub.2 O). 80.degree.-150.degree. C., preferably 90.degree.-130.degree. C. digestion temperature is employed. A desilication seeding product is added to enhance desilication of the digestion liquor. The digestion residue is separated, and the alumina hydrate is precipitated as is customary in the Bayer process. A drawback of this Grubbs process is that it employs a digestion liquor that is nearly saturated in silica and nearly saturated in alumina. Therefore, must of its digestion capacity is sacrificed, leaving available for digestion only the small difference between the saturation level in alumina, and the actual near saturation level of alumina in the liquor. This results in the unfavorable requirement of huge amounts of digestion liquor to enable the dissolution of sufficient amounts of alumina hydrate.
An unfavorable characteristic of the process widely used for the processing of gibbsitic bauxites is the chemical caustic soda loss caused by the reactive SiO.sub.2 fed into the alumina plant with an Na.sub.2 O/SiO.sub.2 mass ratio of about 0.65 to 0.69. The dissolving potential of the digestion liquor is not utilized completely, so excess heat and power is used for heating and pumping part of the liquor flow. This also reduces the capacity of the digestion plant unit and increases the specific energy consumption. Another disadvantage of the typical low-temperature digestion process is the retention time of 0.5 to 2 hours, which requires the installation of expensive pressure vessels (autoclaves). It is difficult to solve the material handling and to prevent the settling of the solid phase in the autoclaves. Yet another disadvantage of the widely used low-temperature digestion process is the secondary boehmite formation, which often causes Al.sub.2 O.sub.3 losses amounting to 0.5% to 2%. The secondary gibbsite formation in the settling and washing system is also a disadvantage of the present processes, leading usually to Al.sub.2 O.sub.3 losses of 2% to 3%.